As part of the effort to determine the difference of longitude between Washington and Paris, two longitude transit instrument were ordered from G. Prin of Paris. They were to be duplicates of those in use by French observers. There are 3 inches aperture, 33 inches focal length, with self-registering, right ascension micrometers driven by small electric motors and controlled by hand, reversible one each star, with hardened steel pivots which bear throughout their whole length, and electric lighting. A meridian mark and a lens of approximately 160 feet focal length was provided with each instrument. Two small buildings were erected on the Observatory grounds in Washington, DC to shelter the instruments. The astronomical program was to observe from 7:30 p.m. to 1:30 a.m. local mean time at both Washington and Paris. About five clock stars per hour could be observed. For more information, see see United States Naval Observatory Publications, Second Series, Volume 9 (1918), p. E9. USNOA-2021-015 USNO Glass Plate Publication Images

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